Method of synchronizing a mobile station with a base station

ABSTRACT

The invention relates to a method of synchronizing a mobile station with a base station in a wireless communications system, to a mobile station for a wireless communications system, as well as to a wireless communications system of this kind. In order to enable faster ultimate synchronization, it is proposed to compare components of the received data not only with a stored synchronization pattern, but also with a stored identification pattern prior to the adaptation of the timing of the mobile station to received data. Already before the synchronization it can thus be determined with a high degree of probability whether received data originates from a desired base station or not. The necessity of checking the contents of packets which are not associated with a desired base station can thus be avoided. Moreover, because of such preliminary checking, it is no longer necessary either to reset the timing of the mobile station after synchronization on the basis of data which later appear to originate from another base station.

[0001] The invention relates to a method of synchronizing a mobilestation with a base station in a wireless communications system, to amobile station for a wireless communications system, and to a wirelesscommunications system of this kind.

[0002] Wireless communications systems which include at least one basestation and at least one mobile station are known from practice, thatis, notably for wireless communication in locally restricted areas. Anexample of a wireless communications system is the DECT (DigitalEnhanced Cordless Telecommunications) system which is described interalia in ETSI EN 300 175: “Digital Enhanced Cordless Telecommunications,Common Interface”.

[0003] On the one hand, 10 different frequencies are available for thetransmissions in a DECT system. On the other hand, the transmission timeis subdivided into frames, each of which comprises 24 time slots. Eachcombination of a given frequency and a given time slot constitutes aradio channel which can be used for transmissions by a base station or amobile station. In order to enable communication to be started between amobile station and a base station in a DECT system, it is necessary tosynchronize the mobile station first with the base station with which itis associated. To this end, in a first step the mobile station must findthe radio channel used by the relevant base station; in this respect itis to be noted that the mobile station can in most cases receive signalsfrom a plurality of base stations. The mobile station will also transmitat the frequency of said radio channel, be it offset by 12 time slots.The timing of the mobile station can then be adjusted on the basis ofthe signals received via said radio channel in a second step in such amanner that it corresponds to the timing of the base station. Thechanging of the time slots in the mobile station is thus tuned to thechanging of the time slots in the base station.

[0004] For the purpose of illustration FIG. 1 shows a typical radioenvironment of a mobile station on the basis of a co-ordinate system.The radio environment is composed of the signals from various mobilestations and base stations within the range of the mobile stationconsidered. The x-axis of the co-ordinate system is subdivided inconformity with the configuration of the time slots 0 to 23 used by themobile station concerned prior to synchronization. The 10 differentfrequencies 0-9 of the system are plotted on the y-axis of theco-ordinate system. Dark rectangles in the diagram represent radiochannels instantaneously used by base stations and bright rectanglesrepresent channels instantaneously used by mobile stations.

[0005] In the situation illustrated in the figure, transmissions takeplace by ten different base stations, each station utilizing a differentchannel. Five of the transmissions by base stations as shown form partof a duplex connection which is associated each time as a respectivesecond part with the mobile station transmissions which are also shown.The mobile stations transmit at the same frequency as the associatedbase station, but 12 time slots after said base station. The channelsshown are allocated only approximately a given time slot on the x axis,because the time slots used by the various stations usually do notcorrespond to the time slot of the relevant mobile station for which theco-ordinate system was conceived.

[0006] The mobile station under consideration itself does not use anychannel yet for the transmission. For a synchronization so as toestablish communication, the mobile station must search the radiochannel of the base station associated with the mobile station. It isassumed that in this case this is the channel 11 with the carrierfrequency 8 in the vicinity of the time slot 4 of the mobile station.

[0007] A procedure which is known from practice will now be describedfor the determination of this channel.

[0008] The mobile station first measures the received signal strength,notably a RSSI (received signal strength indicator) on all 240 channelsin question. Subsequently, it activates its receiver successively foreach of the channels, that is, in the order of the signal strengthsmeasured and starting with the channel with the strongest signal. In theenvironment shown in FIG. 1 at least 15 channels exhibit a signalstrength that can be measured and must, therefore, be tested by themobile station. Because the time slots of the relevant station do notcorrespond to those of the searching mobile station, measurable signalstrengths may also be detected partly in two time slots of the searchingmobile station, so that the total number of channels to be tested maybecome larger than the number of actual transmissions.

[0009] The signals transmitted by a base station or a mobile station andreceived by the searching mobile station in the DECT system have theform of a DECT packet which is shown in FIG. 2, be it not to scale. Itis composed of a preamble 21, a synchronization word 22 of 16 bits, an Afield 23 and possibly a B field 24. When the searching mobile stationreceives a synchronization word 22, it can adapt its local timing tothat of the bit and time slot timing of the DECT packet, known on thebasis of the word, and hence to that of the transmitting station.

[0010] Therefore, for the synchronization the mobile station has tofilter packets of the base station associated with the mobile stationfrom the signals received. To this end, the mobile station utilizes acomparison circuit whereby packets of base stations can be recognized onthe basis of the synchronization word 22. The synchronization word 22has a value of 0xE98A in packets of DECT base stations and a value of0×675 in packets of DECT mobile stations.

[0011]FIG. 3 is a diagrammatic representation of such a comparisondevice. It includes a 16-bit shift register 31, a comparison circuit 32,and a memory 33 of the mobile station in which the value of the 16-bitsynchronization word 22 in packets of base stations is stored. Inputs ofthe comparison circuit 32 are connected to outputs of the shift register31 as well as to outputs of the memory 33. The input of the shiftregister 31 is also connected to the receiver of the mobile station (notshown) and the output of the comparison circuit 32 is connected to asynchronization device which is not shown either. The memory 33 need notform part of the comparison circuit itself; it may also merely beconnected to the comparison device.

[0012] The data bits 34 which are received at the instantaneousfrequency by the receiver of the mobile station are shifted through the16-bit shift register 31. After each bit 34 newly shifted in, all 16bits present in the shift register 31 at that instant are transferred tothe comparison circuit 32. The comparison circuit 32 also receives the16-bit synchronization word stored in the memory 33. The respective 16bits received are then compared in the comparison circuit 32. When thetwo entries correspond, an output signal 35 which indicates that asynchronization word of a base station has been found is generated.

[0013] In order to compensate the deviations in the time slot timing,the receiver and the comparison device of the mobile station areactivated for a duration of two time slots for each channel. It is thusensured that each synchronization word transmitted by a base station isrecognized by the comparison device in the mobile station.

[0014]FIG. 4 illustrates the detection of a synchronization word of abase station over two time slots. In the Figure the activation and thedeactivation of the receiver and the comparison device of the mobilestation and a received DECT packet 41 are associated with the time slotsof the mobile station.

[0015] At the beginning of a first time slot N−1 both the receiver andthe comparison device of the mobile station are activated. The receiverreceives a DECT packet 41 at the adjusted frequency, which packetcommences inside the first time slot N−1 and also extends into thesecond time slot N because of the different timing of the transmittingstation and the mobile station in question.

[0016] If no synchronization word of a base station is recognized withintwo complete time slots N−1, N of the mobile station, because either nopacket is received at all in these time slots or only a packet ofanother mobile station, the receiver and the comparison device aredeactivated again at the end of the second time slot N. Subsequently,the further search takes place in the two time slots in which thechannel with the next-higher measured signal strength may be situated.

[0017] However, as soon as data bits are received for which a comparisonwith the stored synchronization word by the comparison device indicatesthat the synchronization word associated with the base stations isconcerned, the comparison device is deactivated again. Subsequently, thereceiver is deactivated, after complete reception of the DECT packet 41with which the synchronization word received is associated, inconformity with the timing of the synchronization word and hence of thetransmitting base station; this means that deactivation does not have towait for the end of the second time slot N. The deactivation of thereceiver brings forward the beginning of the next time slot N+1, so thatthe local timing of the mobile station is adapted to that of the basestation.

[0018] Granted, the timing of the mobile station is now adapted to thetiming of the base station wherefrom the received DECT packet 41originates. However, the wrong base station may still be involved, sothat prior to the start of communication an identity of the transmittingbase station, contained in the A field of the packet 41 received, mustbe tested by appropriate software of the mobile station.

[0019] When the software test reveals that a packet 41 of the basestation which is associated with the mobile station is concerned, themobile station retains the adjusted synchronization and thecommunication may commence.

[0020] However, if the software test reveals that the packet 41 receivedis a packet from another base station, the previous timing of the mobilestation is restored and the search for a packet with a synchronizationword from a base station commences anew. The execution of such a newsearch is shown in FIG. 5 in which the activation and deactivation ofthe receiver and the comparison of the mobile station as well as thereceived DECT packet 41 are again associated with the time slots of themobile station. The further search in this case does not commence withthe next time slot N of the list. The receiver instead is activated inthe same time slot N−1 as in the preceding search. The comparisondevice, however, is activated only at an instant in the time slot N−1 asfrom which the already detected synchronization word from the wrong basestation cannot be detected again. Thus, only synchronization words whichappear later than the initially recognized synchronization word in thistime slot are found. Such repeated testing of the same time slot isnecessary because not all-base stations transmit a B field in the DECTpacket 41, so that the packets may be significantly shorter than a timeslot. For example, up to 9 packets may occur in a period of two timeslots.

[0021] Each of the described steps requires time and hence increases thetotal amount of time elapsing before the mobile station is synchronizedwith the correct base station. In case the searched base station withthe channel 11, determined by the frequency 8 and the time slot 4, istested as the last one in the situation shown in FIG. 1, overall atleast ten base stations must be checked. For the first nine checks asubsequent reset of the mobile station to the original timing isrequired, followed each time first by a renewed attempt in the same timeslot. Moreover, five signals from mobile stations are checked. For thesesignals, however, the correct synchronization word is not found alreadyin the comparison device, so that as a result only the time necessary tocheck whether the correct synchronization word is present in the datareceived is added.

[0022] The time required before the ultimate synchronization is achievedis substantially increased in the case of a large number of transmittingstations, because every undesirable signal to be checked requires anadditional amount of time. Thus, it is a drawback of the known method ofsynchronizing a mobile station with a base station in a wirelesscommunications system that the synchronization may require acomparatively long period of time.

[0023] Moreover, the synchronization word has a length of only 16 bitsin DECT systems. The probability of finding this word in noise or in thetransmitted data from base stations or mobile stations, therefore, isnot insignificant, so that the time required for the synchronization isincreased further.

[0024] It is an object of the invention to provide a method, a mobilestation and a wireless communications system which enable fastersynchronization of a mobile station with a base station in a wirelesscommunications system.

[0025] On the one hand, the object is achieved by means of a method ofsynchronizing a mobile station with a base station in a wirelesscommunications system, which method includes the following steps:

[0026] transmission of messages by the base station via a given channelwhich is defined by the frequency and the time slot, at least part ofthe messages containing a synchronization component and anidentification component which identifies the base station,

[0027] reception of one of the messages by the mobile station,

[0028] comparison of components of the received message with a storedsynchronization pattern and comparison of at least one component of thereceived message with a stored identification pattern, and

[0029] synchronization of the timing of the mobile station with thetiming underlying the received message if both comparisons have yieldedcorrespondence.

[0030] On the other hand, the object is also achieved by means of amobile station for a wireless communications system. The mobile stationcomprises first of all storage means for storing a synchronizationpattern and an identification pattern which identifies a given basestation of the wireless communications system. It also includes areceiving device for receiving messages at a selected frequency. Themobile station also includes a comparison device which is connected tothe storage means and the receiving device in order to comparecomponents of a received message with a synchronization pattern which isstored in the storage means and to compare at least one component of areceived message with an identification pattern which is stored in thestorage means. The mobile station also includes a synchronization devicewhich is connected to the comparison device in order to synchronize themobile station with a synchronization component of a received message ifthe comparison device has detected correspondence between components ofthe message and a stored synchronization pattern and a storedidentification pattern.

[0031] Finally, the object is also achieved in accordance with theinvention by means of a wireless communications system. In addition toat least one mobile station in accordance with the invention, saidcommunications system also includes at least one base station whichincludes a transmission device for the transmission of messages, atleast some of which include a synchronization component and anidentification component which identifies the base station.

[0032] The invention is based on the idea that the search for a basestation by a mobile station in a wireless communications system can besignificantly accelerated when the identity of the mobile stationtransmitting a message is no longer checked after a first tentativesynchronization by a software implementation. Therefore, in accordancewith the invention the identity of the base station transmitting amessage is checked in the same processing step as the acquisition of avalid synchronization component in a message received, that is, prior tothe actual synchronization of the mobile station. The invention thusavoids incorrect synchronizations and, with every incorrectsynchronization avoided, the necessity of resetting the timing of themobile station and of searching for suitable synchronization componentsin received data in the same time slot again is also avoided. Moreover,in conformity with the invention it is no longer necessary to check, byway of a software implementation and after the synchronization, thecontents of data packets which do not belong to a desired base station,so that repeated checking of time slots is also avoided in the casewhere a plurality of packets occurs within a single time slot. Overall,the invention thus enables a reduction of the time required forsynchronization.

[0033] Granted, for example, in DECT systems each transmitted packetdoes not contain an identification component. From the cited ETSIdocument, however, it appears that an identification is contained with aminimum probability of 6.25% in a packet transmitted by a base station,that is, when the base station must transmit paging information andsignalization in each frame, and with a maximum probability of 93.75%,that is, when the base station does not have to transmit paginginformation and signalization. A statistical evaluation of DECT packetsin a Philips semiconductor DECT system has revealed that approximately89% of all packets transmitted by a base station are identity messages.Such messages present can thus be evaluated already prior to asynchronization, thus reducing the overall time required on average forsynchronization.

[0034] A further advantage of the invention resides in the fact thatwhen a synchronization component as well as an identification componentare compared, a larger component of the message must correspond tostored patterns than when merely a synchronization component iscompared. The probability of the pattern stored being present by chancein noise or in data from an arbitrary message is thus reduced. Theprobability that a 16-bit synchronization word is discovered in noisethen is once in 65,536 tested bits and hence once in 56,888 ms ofcontinuous testing. Because the search requires at least 100 ms in alltime slots, this is a negligibly small effect. However, in the case ofan additionally tested identification component of 16 bits, theprobability of discovering the combination of the synchronization wordand the identification component in the noise is only once in 4294967296bits and hence once in 62 minutes and 8 seconds of uninterruptedtesting.

[0035] At the same time the testing of a synchronization component aswell as an identification component suffices, with a high degree ofprobability, for the identification of the correct base station.Assuming a tested 16-bit identification component, on average a basestation will have the same 16 bits in the identification component asone in 65536 other base stations. Because the range of the base stationis small, that is, a few hundred meters, the probability of encounteringtwo base stations with the same tested identification component in thesame geographical environment, therefore, is negligibly small, eventhough of course steps must also be taken for this case.

[0036] Advantageous embodiments of the invention are disclosed in thedependent claims.

[0037] The features of the claims 2 to 9 ensure that, in addition to thetesting of the identity of a transmitting base station in accordancewith the invention, conventional (notably software-based) testing of theidentity of a transmitting base station is also possible. This isimportant in the case where the mobile station is not yet associatedwith a given base station or the stored identity component istransmitted only infrequently by the base station. It is thus ensured,preferably by way of a switching-over facility, that a channel can befound also in such cases. The synchronization can then take placeexclusively on the basis of correspondence between a signal componentand the stored synchronization pattern, and a further evaluation of thesignals received can subsequently take place in software of the mobilestation.

[0038] If the assignment of the synchronization component and theidentification component to one another in a message is defined, asimultaneous comparison with the stored synchronization pattern and thestored identification pattern could take place, for example, by way of acorrespondingly long shift register into which the bits of the messagereceived are sequentially entered and which is capable of applying allbits received to a comparison circuit in one operation. Preferably,however, in conformity with the features of the claims 3 and 8 twocomparison means are used for the comparison of the two patterns. Thisoffers the advantage that the confirmation of the correspondence of thesynchronization component is separately available for further use. Sucha separate confirmation is used to check given radio signals such asso-called “bit slicers”. In that case the comparison means may includenotably a shift register and a comparison circuit which compares therespective instantaneous contents of the shift register with the memorycontents. The result of the comparison means for the component that canbe detected ahead in time of the other component can then be applied ina delayed fashion, via a counter, to an AND element which also receivesthe result of the second comparison means directly. The output value ofthe AND element thus provides information as regards the correspondenceof the two components in a message.

[0039] The characterizing part of claim 4 offers the advantage that thechecked part of an identification component of a message can be selectedin dependence on the system, for example, by way of different lengths ofthe checked component. This is done notably by checking a header of afield in a message which indicates whether the field contains anidentification of the base station, and that further in the field acomponent of a given length is checked as from a given position.Correspondences in the checked identification component of differentbase stations can thus also be avoided. The identification component ofvariable length is also referred to as a PARK (Portable Access RightsKey) in DECT systems.

[0040] The characterizing part of claim 5 yields a preferred version ofthe synchronization in conformity with the invention. While making onlyminor modifications in the customary method, this version enables thecorrect base station to be found faster in most cases than by means ofthe customary methods. The search for the correct channel in the orderof signal strength of the channels is then based on the assumption thatthe mobile station in most cases wishes to contact one of the basestations which are nearest in space. Even when an identificationcomponent is not included in each message, the correct channel can befound with a very high degree of probability by setting a given numberof repeats for the checking of each channel. The checking of two timeslots of the mobile station per channel then ensures that each messageis considered completely during a comparison.

[0041] The characterizing part of claim 6 yields an alternative,preferred version of the synchronization in accordance with theinvention. For example, in a DECT system most messages are transmittedonly once or only during a short period of time. Messages which are onlyoccasionally transmitted, therefore, normally will not result again infalse correspondence in the case of a second attempt. The only messageswhich are regularly transmitted are Qt messages, that is, except foridentification messages, are Qt messages. These messages, representingsystem messages, however, have a frequency of less than {fraction(1/30)} of the identification messages when two different Qt types areused. When four Qt types are used, the ratio is reduced even to{fraction (1/60)}. Therefore, false correspondences will be recognizedonly in isolated cases. The probability of recognition of a falsecorrespondence twice in succession, therefore, is negligibly small. Thiseffect can again be completely avoided by utilizing the proposed PARKrecognition.

[0042] The second preferred version, therefore, enables a significantacceleration to be achieved. Even when the base station only rarelytransmits an identification, which is not customary, the speed willstill be satisfactory. Because the signal strength is not taken intoaccount, the detection will take place without influencing by othersystems.

[0043] A comparison device implemented on the basis of software is alsofeasible when the comparison in a comparison device is preferablycarried out on the basis of hardware.

[0044] These and other aspects of the invention will be described indetail hereinafter with reference to the embodiments and the drawings.Therein:

[0045]FIG. 1 shows a typical radio environment for a DECT mobilestation,

[0046]FIG. 2 shows the format of a DECT packet,

[0047]FIG. 3 shows diagrammatically a customary comparison device,

[0048]FIG. 4 shows an example of successful detection of asynchronization word,

[0049]FIG. 5 shows an example of a repeated attempt for the detection ofa synchronization word,

[0050]FIG. 6 is a diagrammatic representation of a comparison device ofan embodiment of a mobile station in accordance with the invention,

[0051]FIG. 7 shows the format of a DECT A field, and

[0052]FIG. 8 is a diagrammatic representation of an embodiment of acomparison means of a comparison device for the detection of theidentity of a base station in conformity with the invention.

[0053] The FIGS. 1 to 5 have already been described in the preamble ofthe present description.

[0054] The embodiments to be described hereinafter are based on a DECTsystem in accordance with the invention which includes a mobile stationin accordance with the invention which is provided with a receivingdevice, storage means, a comparison device and a synchronization device.

[0055] First the comparison device of the mobile station will bedescribed in detail. This comparison device is shown diagrammatically inFIG. 6.

[0056] The comparison device includes first comparison means 61 andsecond comparison means 62. The input of the first comparison means 61and the input of the second comparison means 62 are connected inparallel to the receiving device (not shown) of the mobile station. Theoutput of the first comparison means 61 is connected, via a counter 63,to a first input of an AND-gate 64 and the output of the secondcomparison means 62 is connected directly to a second input of theAND-gate 64. The output of the AND-gate 64 in its turn is connected tothe synchronization device (not shown) of the mobile station.

[0057] Both comparison means receive the data bits 65 which are receivedby the receiving device of the mobile station. When such data bits 65originate from a DECT base station, they form part of packets whichcorrespond to the DECT packet shown in FIG. 2; the B field 24 may bepresent in some of the packets and absent in other packets. In the fieldprovided for this purpose the packets contain a synchronization word 22and, moreover, in approximately 89% of the packets an identificationword in the A field 23, which identification word identifies the basestation wherefrom the packet originates.

[0058] When the mobile station wishes to start communication with a basestation, first the first comparison means 61 are activated. The databits 65 received are compared with a synchronization pattern which isstored in the mobile station and has the value 0xE98A and hence isidentical to the synchronization word of the DECT packets originatingfrom base stations. As soon as a sequence of data bits which correspondsto the stored synchronization pattern is recognized, the counter 63 isstarted and the subsequent bits 65, originating from the A field of thepacket, are applied to the second comparison means 62. When the counter63 reaches a predetermined value, the sequence of data bits 65 thenpresent in the second comparison means 62 is compared with anidentification pattern which is stored in the mobile station. Because ofthe presetting of the comparison point by the counter 63, the secondcomparison means 62 need perform only a single comparison.

[0059] The result of the comparison in the first comparison means 61 andthe result of the comparison in the second comparison means 62 areapplied to the AND-gate 64. The supply of the result of the comparisonin the first comparison means 61, however, is delayed by the counter 63,so that the two results for a packet simultaneously reach the AND-gate.When the instantaneous data bits 65 in the second comparison means 62correspond to the identification pattern, via the AND-gate 64 an outputvalue 66 can be output which indicates the correspondence of thesynchronization word as well as of the compared identification componentin the instantaneous packet with the stored patterns. This triggerssynchronization of the mobile station with the synchronization componentof the received packet by the synchronization device. Moreover, thesoftware of the mobile station is informed on the positive result. Whenthe instantaneous data bits 65 do not correspond to the identificationpattern, the AND-gate 64 outputs an output value 66 which indicates thatno double correspondence has been found. Subsequently, the firstcomparison means 61 resume the search for a synchronization word.

[0060] As an alternative for two separate comparison means 61, 62, asingle comparison of a correspondingly longer bit sequence could takeplace. A comparison split into two parts, however, offers the advantagethat it utilizes the detection of synchronization words only for thecontrol of given radio signals. A separate signal which indicatesexclusively that a synchronization word has been found, therefore, isrequired in any case.

[0061] When the synchronization device receives the information that thecomparison was successful, it synchronizes the mobile station in knownmanner in conformity with the synchronization word. Subsequently, thepacket obtained is checked in software whereby, for example, the rarecases can be detected where both the synchronization word and thechecked identification component appeared in noise or in data fromanother base station. At that instant it can also be checked whether thepacket does not originate from the base station associated with themobile station, but from another base station with the sameidentification component. If no incorrect detection is found in thesoftware either, the adjusted synchronization is maintained and thecommunication between the mobile station and the base station maycommence. When incorrect detection is recognized, the timing of themobile station must be reset and a search must be undertaken once morefor a data packet from the correct base station. Such incorrectdetection, however, is so rare that it does not have a significanteffect on the mean synchronization time.

[0062] Instead of using exclusively a recognized synchronization word soas to trigger the synchronization and subsequently checking the identityof the base station transmitting the synchronization word in software,the mobile station thus is synchronized and the received signal ischecked further only if the synchronization word is found and thedefined component of the identification word is contained in the A fieldof a packet received. This means that packets from incorrect basestations are not checked and that the time usually spent on the checkingand subsequent resetting of the timing of the mobile station can besaved.

[0063] For example, the comparison means which are diagrammaticallyshown in FIG. 3 can be used as the first comparison means 61 for thecomparison of received data bits 65 with the stored synchronizationpattern.

[0064] The second comparison means 62 for the comparison of receiveddata bits 65 with the stored identity pattern may also be constructedlike the comparison means shown in FIG. 3. However, the comparison means62 are preferably constructed to be such that they enable more flexibledetermination of the identity of the transmitting base station.

[0065] In order to illustrate such flexible comparison means, first theincorporation of the identity word in the A field 23 of a DECT packet asshown in FIG. 2 will be described. As is shown in FIG. 7, the A field iscomposed of a header 71 of 8 bits, a data component 72 of 40 bits and aCRC (Cyclic Redundancy Check) component 73 of 16 bits. The 48 bits ofthe header 71 and the data component 72 are protected by the 16-bit CRCcomponent 73. The first three bits in the header 71 indicate the type ofcontents of the data component 72. When an identity message isconcerned, the first three bits of the header 71 have a value “011” andthe 40 bits of the data component 72 contain an identification of thetransmitting base station.

[0066] The identification component of a received packet, checked in thesecond comparison means 62 of FIG. 6, may have an arbitrary length andbe situated in an arbitrary position in the A field in the case of aflexible construction of the comparison means. The identificationcomponent in that case need not be coherent either.

[0067]FIG. 8 shows an advantageous embodiment of flexible comparisonmeans 62 of the comparison device of FIG. 6 for the comparison of theidentity pattern with an identification component from the A field of aDECT packet as shown in FIG. 7.

[0068] The comparison means include a shift register 81 for theintermediate storage of 48 bits a₀-a₄₇ and a comparison circuit 82. Alsoprovided are storage means 83 in which 42 bits are stored. The storagemeans 83 may form part of the comparison means or may be general storagemeans in which further data is additionally stored for other devices ofthe mobile station, notably for the comparison means 61 for thesynchronization word. The input of the shift register 81 corresponds tothe input of the second comparison means 62 of FIG. 6 and hence isconnected to the receiving device (not shown) of the mobile station.Inputs of the comparison circuit 82 are connected to outputs of theshift register 81 as well as to outputs of the storage means 83. Theoutput of the comparison circuit 82 corresponds to the output of thesecond comparison means 62 in FIG. 6 and hence is connected to theAND-gate 64 (not shown) of FIG. 6.

[0069] After a synchronization word has been detected in a received DECTpacket in the first comparison means 61 of FIG. 6, the first 48 bits ofthe A field of the received packet are sequentially shifted into thesecond comparison means 62 of FIG. 6, and hence into the shift register81 of FIG. 8, until all bits of the header and the data component of theA field have been stored in the shift register 81. The instant at whichall bits 65 of the A field will have been written is preset by thecounter 63 of FIG. 6. The first three bits a₀-a₂ written into the shiftregister 81 and the bits a₉ to a₄₇ are then applied in parallel to thecomparison circuit 82. Similarly, the 42 bits stored in the storagemeans 83 are also applied in parallel to the comparison circuit, thefirst three bits having the value “011” while the further bitsconstitute an identification of the desired base station which isidentical to the bits a₉ to a₄₇ in an A field of DECT packets with anidentification component of the desired base station.

[0070] The comparison circuit 82 of the second comparison means 62 thencompares on the one hand the first three bits a₀-a₂ in the header 71 ofthe A field, indicating whether the received data 65 concerns anidentity message, with the corresponding bits from the storage means 83.As opposed to the first three bits in a first section 84 in the shiftregister 81, the six bits a₃ to a₈ in the subsequent section 85 are notapplied to the comparison circuit 82 and hence are not checked. A thirdsection 86 contains a bit sequence of a given length of the datacomponent 72 of the A field; this length may be dependent notably on thetype of system in which the mobile station wishes to communicate. Thisvariable identity component is also known as PARK and is contained inthe bits a₉-a_(n), where n may be between 9 and 47. Because of thevariable length of PARK, the comparison circuit 82 must be capable ofchecking an identity component of variable length. A final section 87with the bits a_(n+1) to a₄₇ again is not checked, the boundary 88between the third section 86 and the fourth section 87 being differentfrom one system to another. In the example shown in FIG. 8, the bits a₄₅to a₄₇ are assigned to the fourth section 87 and hence are ignoredduring the comparison. If correspondence was found between the receivedand the stored bits in the sections 84 and 86, a signal 89 whichindicates such correspondence is output.

[0071] The CRC bits a₄₈ to a₆₃ need hardly be included in the identitycheck, because the probability of correspondence of all identity bitswith the stored pattern while the CRC sum is wrong is small enough toavoid any significant effect on the result.

[0072] The checking of an identity component in received DECT packetsalready before synchronization, of course, makes sense only when themobile station already knows the identity of the base station itsearches. This is not necessarily the case, for example, during a firstpresentation of a mobile station or when the base station has thedesired identity only as a secondary or tertiary identity; in the lattercase the identity can also be transmitted, be it only rarely. In suchcases the conventional synchronization is still necessary, provisionalsynchronization already taking place merely on the basis of asynchronization word found. Therefore, the software of the mobilestation includes an option which is capable of switching over, ifnecessary, between the conventional method and the method in accordancewith the invention.

[0073] Hereinafter a description will be given of two preferredpossibilities for the use of the comparison device of the embodimentshown in the FIGS. 6 and 8 in a mobile station for synchronization witha desired base station when the mobile station is present in the radioenvironment shown in FIG. 1.

[0074] The mobile station must again be synchronized with the basestation which utilizes the channel 11 which results from the combinationof the frequency 8 and the time slot 4 of the mobile station.

[0075] In conformity with a first, preferred alternative thesynchronization is realized as in the case of the conventional methoddescribed with reference to the FIGS. 4 and 5, except for the fact thata preselection of the channels is already carried out by the secondcomparison means 62 of the comparison device of the mobile station.

[0076] Thus, first the signal strength in the form of the RSSI of allpossible channels is measured in the mobile station. Starting from thechannel with the strongest RSSI, a check of packets received via thechannels is then performed in the comparison device of the mobilestation, that is, in the order of the measured RSSI, as described withreference to the FIGS. 6 to 8. The receiving device and the comparisondevice are then activated each time for up to two time slots. After thecorrect synchronization word and the correct identification componenthave been found in a received packet within two time slots, the receiveris deactivated in conformity with the contents of the synchronizationword so that the timing of the mobile station is adapted to the timingof the base station. If no correspondence with the stored patterns isfound within the two time slots, the next channel is checked. If, aftera successful synchronization, it is detected in the software that theselected channel was not the correct channel after all, the same twotime slots are checked again; the comparison then commences at aninstant within the time slot such that the previously incorrectlydetermined packet cannot be detected again.

[0077] Each channel is checked three times in succession as to whetherthe desired synchronization word and the desired identificationcomponent are present therein, that is, for as long as the correctchannel has not been found. Assuming a probability of approximately 89%that an identification word is present in a data packet, the desiredbase station will be recognized with a probability of almost 99.9% whenthree attempts are made during the checking of the correspondingchannel.

[0078] The advantage of this alternative resides in the fact that,because of the second comparison means, the mobile station generallywill not be synchronized with a base station with which it is notassociated. The time required for the checking of channels of other basestations after successful synchronization in other conventional methodsis thus saved. Granted, all channels having an RSSI higher than that ofthe desired base station must still be checked prior to successfulsynchronization as before. However, in the presence of more than onetransmitting base station in the radio environment of the mobilestation, the overall amount of time lost is significantly smaller thanin the case of the conventional method, because neither an identitycheck by the software nor a regular resetting of the timing of themobile station is required. This alternative represents an extension ofthe customary method which can be simply realized and implemented withonly a small amount of work and coding.

[0079] Because the first alternative is based on the RSSI of thereceived signals, the number of base stations in the system influencesthe synchronization time. For example, when the desired base station isnot the base station with the strongest RSSI, time will be required tocheck first the channels having the stronger RSSI. In the case of threeattempts per channel, it may be assumed that a time of approximately 30ms will be required per channel.

[0080] Instead of each time two time slots per channel, in conformitywith a second preferred alternative the mobile station utilizes a widereceiving window over 25 time slots each time for all channels of acarrier frequency. To this end it is assumed that the respectiveadjusted frequency of the receiver of the mobile station remains stablefor at least 25 time slots and hence for 10,416 ms. Customarily this isthe case only for receivers which operate in a so-called closed loopmode. The window is positioned in such a manner that it covers all 24possible channels of a frequency. Once more synchronization is carriedout only when, using the comparison device described with reference tothe FIGS. 6 to 8, a DECT synchronization word of a base station has beenfound as well as the desired identification component in the subsequentA field of a received packet.

[0081] If no correspondence is found for a carrier frequency, the nextcarrier frequency is checked.

[0082] When a packet with the correct synchronization word and thecorrect identification component is found for a carrier frequency, themobile station awaits the next identity message on this channel andcarries out a new check so as to make sure that the message indeedoriginated from the correct base station. If it is found that it was notthe correct base station and that the values correspond only by chance,the search is continued with a further window which commences exactly atthe end of the synchronization word of the packet of the wrong basestation. This window, however, terminates at the same time slot positionas the original window; this means that it is shorter than 25 timeslots. The continuation of the search after a packet has unduly beenconsidered to be correct thus corresponds to the procedure according tothe first alternative in the case of incorrectly recognizedcorrespondence, be it that in this case 25 time slots are used insteadof two.

[0083] In this case the order of the strengths of measured RSSI is notchecked; a sequential check of the 10 frequencies available is performedinstead.

[0084] The actual synchronization again takes place by deactivation ofthe receiver at an instant dictated by the correct synchronization word.

[0085] The synchronization time required hence is dependent on thefrequency at which the identification message searched is transmittedand on the probability of recognition of incorrect synchronizations.

[0086] Calculations have shown that the second alternative with thecomparison device in accordance with the invention can lead to asignificant acceleration of the synchronization in typicalcircumstances. Even in extreme circumstances, in which the base stationstransmit information messages only rarely (being an exceptional case),the performance is still satisfactory. Because an identity component isalso detected prior to the actual synchronization and the evaluation ofRSSI is dispensed with in accordance with the invention, the method inconformity with the second alternative is not subject to any significantinfluencing by other systems. The second alternative requires a morecomplex conversion than the first alternative, but offers a furtherenhancement of the performance.

[0087] The invention has been described for the use in a DECT system andfor the packet formats customarily used in DECT systems. However, it canbe used equally well for other wireless communications systems and forother packet formats.

1. A method of synchronizing a mobile station with a base station in awireless communications system, which method includes the followingsteps: transmission of messages by the base station via a given channelwhich is defined by the frequency and the time slot, at least part ofthe messages containing a synchronization component and anidentification component which identifies the base station, reception ofone of the messages by the mobile station, comparison of components ofthe received message with a stored synchronization pattern andcomparison of at least one component of the received message with astored identification pattern, and synchronization of the timing of themobile station with the timing underlying the received message if bothtwo comparisons have yielded correspondence.
 2. A method as claimed inclaim 1, characterized in that the comparison of at least one componentof the received message with a stored identification pattern, or thetaking into account of a comparison of at least one component of thereceived message with a stored identification pattern, can be prevented,the timing of the mobile station then being synchronized with the timingunderlying the received message if the comparison of components of thereceived message and the synchronization pattern stored has yieldedcorrespondence.
 3. A method as claimed in claim 1, characterized in thatcomponents of the received message are compared, separately from oneanother, with the stored synchronization pattern and with the storedidentification pattern, the results of the individual comparisons beingcombined so as to form an overall result for the synchronization.
 4. Amethod as claimed in claim 1, characterized in that at least onecomponent of the received message is compared with selectable parts ofthe identification pattern stored in the mobile station.
 5. A method asclaimed in claim 1, characterized in that first the strength of receivedsignals is measured separately for all relevant channels, thecomparisons for each channel being carried out in the order of thesignal strengths associated with the channels, starting with the channelhaving the strongest signal and each time with a predetermined number ofattempts, until correspondence is found for message components with thestored synchronization pattern as well as with the stored identificationpattern, the comparisons for each channel being based on the signalsreceived within two time slots of the mobile station.
 6. A method asclaimed in claim 1, in which the mobile station carries out thecomparisons in a time window which comprises all relevant time slots,the comparisons are sequentially carried out in the time window for allrelevant frequencies for as long as no correspondence is found for afrequency, when correspondence is found, new comparisons with componentsof a subsequent message are carried out for the same frequency and inthe same position of the time window, a synchronization of the mobilestation in conformity with the synchronization component of the messagetakes place when the new comparisons reveal correspondence again, andwhen the repeated comparisons do not demonstrate correspondence, awindow for further comparisons is positioned in such a manner that itcommences directly after the beginning of the synchronization componentof the first message compared in the same position and that it ends atthe end of the time window used thus far.
 7. A mobile station for awireless communications system, which station includes storage means forstoring a synchronization pattern and an identification pattern whichidentifies a given base station of the wireless communications system, areceiving device for receiving messages at a selected frequency, acomparison device which is connected to the storage means and thereceiving device in order to compare components of a received messagewith a synchronization pattern which is stored in the storage means andto compare at least one component of a received message with anidentification pattern which is stored in the storage means, and asynchronization device which is connected to the comparison device inorder to synchronize the mobile station with a synchronization componentof a received message if the comparison device has detectedcorrespondence between components of the message and a storedsynchronization pattern and a stored identification pattern.
 8. A mobilestation as claimed in claim 7, characterized in that the comparisondevice includes separate comparison means for the comparison ofcomponents of a received message with a stored synchronization patternand for the comparison of at least one component of a received messagewith a stored identification pattern, each of the comparison means beingarranged to output an output value which indicates whethercorrespondence with the respective pattern has been detected, the mobilestation being provided with logic means which are arranged to combinethe output values of the two comparison means for each message so as toform a combined output value which indicates whether correspondence hasbeen detected with the stored synchronization pattern as well as withthe stored identification pattern.
 9. A mobile station as claimed inclaim 8, characterized in that it includes switching means which arearranged to supply the synchronization device either with the combinedoutput value of the two comparison means or only with the output valueof the comparison means for the comparison of components of the receivedmessage with a stored synchronization pattern, the synchronizationdevice being suitable to determine for both cases, on the basis of theoutput value received, whether the message is to be used forsynchronization of the mobile station.
 10. The use of a method asclaimed in one of the claims 1 to 6 or of a mobile station as claimed inone of the claims 7 to 9 in a DECT (Digitally Enhanced CordlessTelecommunications) system.
 11. A wireless communications system whichincludes at least one base station and at least one mobile station, thebase station including a transmission device for the transmission ofmessages, at least part of which includes a synchronization componentand an identification component which identifies the base station, themobile station including storage means for storing a synchronizationpattern and an identification pattern which identifies a given basestation of the wireless communications system, a receiving device forreceiving messages at a selected frequency, a comparison device which isconnected to the storage means and the receiving device in order tocompare components of a received message with a synchronization patternwhich is stored in the storage means and to compare at least onecomponent of a received message with an identification pattern which isstored in the storage means, and a synchronization device which isconnected to the comparison device in order to synchronize the mobilestation with a synchronization component of the received message if thecomparison device has detected correspondence between components of themessage and the stored synchronization pattern and the storedidentification pattern.
 12. A wireless communications system as claimedin claim 11, characterized in that it is a DECT (Digitally EnhancedCordless Telecommunications) system.